In RF signal monitoring systems, it is desirable to have some means for detecting and indicating when a signal being monitored contains transmissions at a particular frequency or frequencies. A Bragg cell is utilized with success for this purpose. Such monitoring is based on the characteristic of a Bragg cell to deflect a light beam passing therethrough as a function of an acoustical signal also passed through the cell. A narrow coherent light beam provided by a laser is directed toward and through the Bragg cell. The RF signal being monitored is connected to a transducer incorporated in the cell which causes an acoustical wave to be passed through the cell as a function of the RF signal input thereto.
The RF signal will cause an acoustical wave which will deflect the coherent light into a first order beam. If the RF signal contains more than one frequency component, the first order beam is deflected into more than one beam of coherent light. The majority of the beam emerges as an undeflected beam which is absorbed by a stop. Only about 1% of the beam is diffracted as the first order beam. In typical prior art systems, the first order beam is focused by a lens onto a sensing device which may comprise a plurality of photodetectors, especially if more than one frequency constitutes the RF signal. The exact position of the focused, first order beam upon the photodetector is a function of the RF signal frequency. The higher the frequency, the greater the angle of diffraction. The intensity of this first order beam is related to the intensity of the input signal. By identifying the various beam locations on the surface of the photodetectors, it is possible to detect the presence of a particular frequency transmission entering the Bragg cell.
As mentioned, the first order beam is only a small fraction of the original beam of coherent light from the laser. Consequently, only a weak or erratic signal may be generated in the photodetector device. Therefore, regardless of the electrical amplification which might be applied to an output circuit receiving the signal from a photodetector, the resultant signal and data processed therefrom will be no better than the original signal developed at the photodetector.